1. Field of the Invention
The present invention relates to a method for stabilizing coal wastes and to the products that result, and more particularly to a method for stabilizing coal wastes for use as an environmentally stable fill material.
2. Description of the Related Art
Almost one-third of the coal produced in the United States is processed to remove ash and/or sulfur prior to marketing and use. The processing wastes that result from this processing include coarse coal refuse and fine coal processing waste. The coal processing waste (CPW) constitutes about 10%-30% of the run-or-mine coal and, when the processing wastes occur as by-products of desulfurization operations, it can contain particularly high levels of sulfur. Disposal of these wastes is costly and poses long-term environmental problemsxe2x80x94including acid mine drainage, erosion and sedimentation, lack of vegetative growth, negative impacts on aquatic life, and objectionable aesthetic appearance of disposal areas. It is estimated that the present disposal costs for coarse and fine coal processing wastes adds a significant amount (estimated at 3% to 5%) to the total cost of coal delivered to the customer.
The large majority of coal that is produced in the United States is combusted for the generation of heat, steam and power. The combustion of coal results in the generation of coal combustion by-products (CCB""s), such as fly ash, bottom ash and scrubber sludge. When typical Midwestern U.S. coal is combusted, it is estimated that the amount of fly ash and bottom ash that are formed are about 8% and 2%, respectively, of the weight of the coal that is combusted. In addition, scrubber sludge is formed in the amount of about 5% to 10% of the weight of the coal combusted. Thus, besides the coal processing wastes, which are more-or-less simply coal particles that are too small to handle and market, the burning of coal causes the additional burden of coal combustion by-products that must be disposed of.
Typical coal combustion by-products possess several characteristics that can cause environmental problems if the materials are not disposed of carefully. The materials are often alkaline and high pH leachate from fly-ash ponds, or unsealed landfill areas, can cause unacceptable environmental damage. Moreover, such materials can be the source of unacceptable levels of harmful leached elements, such as iron, boron, chromium, lead, selenium and zinc, among others.
Due to the large scale of the coal mining industry and the importance of coal as an energy source, a number of methods for managing these coal combustion by-products have been devised. For example, blends of fly ash with a number of diverse materials have been reported for the purpose of producing soil amendments or fertilizers. U.S. Pat. No. 5,451,240 to Trowbridge discloses the combination of coal ash with humate ore (identified as an oxidized form of lignite coal) in which humic acids have been activated, as by addition of an alkali. The resulting product was prilled and is reported to have excellent plant growth stimulant abilities.
U.S. Pat. No. 5,468,277 to Rehbein et al. discloses mixing a sulfate-containing coal ash, such as fluidized bed combustion (FBC) ash, with water and applying the slurry to soil to elevate the pH of the top layer of soil and working the soil to distribute the ash into the soil either before or after the material hardens.
U.S. Pat. No. 5,277,826 to Bums et al. teaches a method to stabilize waste water treatment sludge by mixing it with fly ash and a source of lime. The product has either soil-like form or load-bearing form. U.S. Pat. No. 5,468,276 to Roth et al. discloses the production of fertilizer chips by blending coal fly ash with manure and compressing the material into a sheet, which is fractured into chips that are useful as fertilizer. U.S. Pat. No. 4,541,857 to Snively teaches that a plant growth inducing compound can be produced by mixing particulate coal with a source of sodium molybdate. Other references, such as U.S. Pat. Nos. 4,013,440; 4,469,503; 4,526,607; 5,248,327; 5,302,524; 5,501,973; and 5,627,133 provide yet more methods to use and/or safely dispose of one or more coal combustion by-products.
Notwithstanding the progress that has been made in the methods for disposing and using wastes resulting from the mining and use of coal, there is still an unmet need for a method to manage coal processing wastes and coal combustion byproducts economically. It would be particularly useful if a method could be found that would derive some value from such wastesxe2x80x94especially if this could be done without the addition of some other, more valuable, material, or without the expenditure of a great amount of energy. It would be even more useful if the management technique would provide a valuable product from the coal wastes that could be used in a relatively short time and without requiring an inordinately large amount of space. Finally, it would be particularly useful if a useful management method could be found that would mitigate the harmful environmental impact of the coal wastesxe2x80x94such as acid drainage and leaching of harmful metals.
Briefly, therefore, the present invention is directed to a novel fill material mix comprising coal processing waste, at least one coal combustion by-product and a neutralizing/stabilizing by-product that is the same as or different than the coal combustion by-product.
The present invention is also directed to a novel mortar comprising a mixture of coal processing waste, at least one coal combustion by-product, a neutralizing/stabilizing by-product that may be the same as or different than the coal combustion by-product, and water in an amount sufficient to permit curing of the mixture.
The present invention is also directed to a novel fill material comprising the cured product of a mixture of coal processing waste, at least one coal combustion by-product and a neutralizing/stabilizing by-product and water in an amount sufficient to permit curing of the mixture.
The present invention is also directed to a novel method for stabilizing coal processing wastes comprising intermixing water, coal processing waste, at least one coal combustion by-product and a neutralizing/stabilizing by-product that may be the same as or different than the coal combustion by-product, in proportions sufficient to cause the mixture to have an acid/base potential of 1/1, or lower.
Among the several advantages found to be achieved by the present invention, therefore, may be noted the provision of a method to manage coal processing wastes and coal combustion wastes economically and, particularly, a method that derives value from such wastes without the addition of some other, more valuable, material, and without the expenditure of a great amount of energy; the provision of a valuable product from the coal wastes that can be used in a relatively short time and without requiring an inordinately large amount of space; and the provision of a method that permits the mitigation of the harmful environmental impact of the coal wastesxe2x80x94such as acid drainage and leaching of harmful metals.